In image forming apparatuses such as a multifunction color copying apparatus (a multifunction peripheral (MFP)), types of media on which color images and the like are printed are increasing. Even if printing media are limited to paper, various kinds of paper with different thicknesses and the like are used.
Such types of paper are usually distinguished by basis weight (unit; g/m2) indicating weight per a fixed area. For example, types of paper belonging to groups of basis weights 64 to 105, 106 to 163, 164 to 209, 210 to 256, and 257 to 300 are referred to as plain paper, thick paper 1, thick paper 2, thick paper 3, and thick paper 4, respectively. The basis weights depend on the density and the thickness of paper. When the density of the paper is fixed, the basis weights are proportional to the thickness of the paper.
Usually, the basis weights are written on a package of paper. When a user selects one of the groups of basis weights written on the package, a printing condition corresponding to a type of the paper is automatically set.
However, many users do not notice the basis weights written on the package of paper. After the paper is taken out from the package, it is extremely difficult for a general user to learn the basis weights.
Therefore, in recent years, it is attempted to save labor and time of a user by causing a media sensor or the like set in an apparatus to select paper.
Usually, the media sensor distinguishes a paper type by detecting characteristics of paper such as the thickness, the light transmittance, and the like of the paper. To accurately detect the characteristics of the paper, it is preferable to separate pieces of the paper one by one. However, a user feels it annoying to sort out only one piece of paper from a bundle of paper and cause the media sensor to detect characteristics of the paper. Therefore, the media sensor is set along a so-called paper path after a bundle of paper stacked in a paper feeding cassette is separated into one piece of paper. Then, a paper type cannot be distinguished after the one piece of paper reaches the media sensor, i.e., until the paper is separated into one piece of paper immediately before printing.
In general, in an apparatus that forms an image and prints the image, it is preferable to adjust an image forming condition according to characteristics of paper types. It is desirable that a paper type is identified until image formation is started. However, in an apparatus that forms a color image and prints the color image, usually, image formation is performed before one piece of paper is separated from a paper bundle. Therefore, even if a paper type is distinguished at this stage, it is difficult to replace paper of some kind in a printing state with another kind of paper. Even if the paper in the printing state can be replaced with another kind of paper, it is necessary to stop traveling of the paper in the printing state and bring another piece of paper into a printing state. This deteriorates performance of printing.
Therefore, it is proposed to detect characteristics of paper in a state in which a paper bundle is stacked in a paper feeding cassette. For example, in JP-A-2005-104723, a CCD sensor is attached to a position opposed to a side end face of a paper bundle set in a paper feeding tray (a paper feeding cassette) of an image forming apparatus and an image of the side end face of the paper bundle is picked up by this CCD sensor. In this laid-open application, a paper feeding device that detects the thickness and the like of paper from the picked-up image is disclosed.
However, in this method, since shading caused by the paper itself and spaces among pieces of the paper is used, light and shade of the shading hardly occurs and it is difficult to accurately measure the thickness and the like of the paper.
It is described to irradiate light of a light source on the side end face of the paper bundle from obliquely above or obliquely below the side end face in order to increase a difference in the light and shade of the shading. However, when side end positions of the paper bundle are accurately aligned, the difference in the light and shade hardly occurs. On the other hand, when side end positions of the paper bundle are not accurately aligned, fluctuation occurs in the length of the shading and it is difficult to accurately detect the thickness and the like of the paper. Besides, the laid-open application also discloses that up and down movement of the paper bundle is used and the air is blown against the paper bundle from a side of the paper bundle. However, it is still difficult to accurately measure the thickness and the like of the paper.